There have been conventionally known semiconductor devices including high breakdown voltage DMOS (Double-diffused Metal Oxide Semiconductor) transistors. In one example, a LDMOS (Lateral Double-diffused Metal Oxide Semiconductor) transistor is formed on an n type (n−) semiconductor substrate. The LDMOS transistor includes a p conductivity type (p) base region formed on a front surface of the semiconductor substrate and an n conductivity type (n+) source region formed on a surface of the base region. In addition, an n conductivity type (n) drift region having a concentration higher than that of the semiconductor substrate is formed on the front surface of the semiconductor substrate at the opposite side of the source region with the base region interposed therebetween. An n conductivity type (n+) drain region having a concentration higher than that of the drift region is formed on a surface of the drift region. Further, a gate electrode to cover a portion of the base region via a gate oxide film is formed on the semiconductor substrate. In addition, a LOCOS (Local Oxidation of Silicon) oxide film to cover a portion of the drift region is formed on the semiconductor substrate.
In the LDMOS transistor of the related art, if the thickness of the gate insulating film has a relatively large thickness of 1000 Å to 1500 Å in consideration of a breakdown voltage of the LDMOS transistor, a gate threshold voltage is increased and leads to insufficient performance of the LDMOS transistor. Making the gate oxide film thinner may resolve such drawback. However, since portions other than the base region of the gate oxide film become thinner simultaneously, there may be a problem in that a gate-drain breakdown voltage cannot be maintained. To avoid this problem, widening a formation region of the LOCOS oxide film has been considered to mitigate a gate drain electric field.
However, if the formation region of the LOCOS oxide film is widened to be closer to the base region, a thick portion called a bird's beak of the LOCOS oxide film may contact a channel, which results in poor transistor performance.